In an image-forming unit to which an electrophotographic method is applied, the following intermediate transfer method is known. First, electrical charges are evenly formed on a latent image carrier made of an inorganic or organic photoconductor. An electrostatic latent image is formed using a laser, light-emitting diode light, or the like modulated by an image signal, and then the electrostatic latent image is developed by a charged toner to form a visualized toner image. Subsequently, the toner image is primarily transferred electrostatically to an intermediate transfer belt, after which the toner image on the intermediate transfer belt is secondarily transferred electrostatically to recording paper. The transferred toner image is fixed by application of heat or pressure, thereby reproducing a desired image.
As the recent demands for office automation equipment extend not only to higher speed and image quality, but also to improved durability for key components, it is now indispensable for intermediate transfer belts to have materials designed to stably produce high-quality transfer images over an extended period of time. The development of a technique for producing an intermediate transfer belt having both excellent electrical and physical properties is becoming increasingly important. Excellent electrical properties include, for example, the ability to achieve accurate image transfer in a color image-forming unit, and the ability to prevent variations in resistance due to the transfer voltage, enabling high-quality transfer images to be stably produced over an extended period of time. Excellent physical properties include, for example, little deterioration in flatness due to plastic deformation caused by loads applied in the width direction of the belt, excellent bending durability, and stable operation even after extended use.
A semiconductive belt obtained by adding a conductive filler to a polyimide resin film having excellent mechanical properties and heat resistance is known as a semiconductive belt usable as an intermediate transfer belt.
An example of such a known conductive belt is a conductive polyimide seamless belt obtained by adding conductive carbon black, such as acetylene black or Ketjenblack, to a polyimide resin having a high mechanical strength (see, for example, Patent Document 1).
Semiconductive polyimide resin belts are also known that are produced from a feedstock solution in which a conductive filler is dispersed in a high-molecular-weight polyamic acid solution obtained by reacting 3,3′,4,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine (see, for example, Patent Documents 2 to 4).
However, although these semiconductive polyimide resin belts have sufficient rigidity because of the use of a rigid polyimide resin, the lack of flexibility has posed the problem of susceptibility to cracking from edge portions of the belt after extended use.
Also known are processes for producing a semiconductive polyimide resin belt from a feedstock solution obtained by dispersing carbon black with a pH of 2 to 4 in a high-molecular-weight polyamic acid solution obtained by reacting 3,3′,4,4′-biphenyltetracarboxylic dianhydride with 4,4′-diaminodiphenyl ether having flexibility (see, for example, Patent Document 5); and processes for producing a belt made of a copolymer of 3,3′,4,4′-biphenyltetracarboxylic dianhydride, p-phenylenediamine, and 4,4′-diaminodiphenyl ether (see, for example, Patent Document 6).
As in the processes of Patent Documents 5 and 6, the use of 4,4′-diaminodiphenyl ether having flexibility as a diamine compound reduces rigidity and thus imparts flexibility, thereby increasing the resistance to cracking from edge portions of the belt. However, the insufficient rigidity causes plastic deformation of the belt by loads applied in the width direction after extended use, resulting in a deterioration in flatness. This deterioration in flatness is known to cause problems such as white patches and color image positioning errors when the belt is used as an intermediate transfer belt in an image-forming unit.
Furthermore, processes for producing a belt made of a copolymer of tetracarboxylic dianhydrides, which are 3,3′,4,4′-biphenyltetracarboxylic dianhydride and pyromellitic acid dianhydride; and aromatic diamines, which are p-phenylenediamine and diaminodiphenylsulfone, are known (see, for example, Patent Document 7).
Rotational molding has been previously considered as a process for producing a seamless belt of a semiconductive polyimide resin with high accuracy. According to Patent Documents 2 to 7, such belts are produced by a process wherein a polyimide resin precursor solution is applied to the inner surface of a cylindrical mold, and a film is formed by centrifugal molding. Next, the removal of a portion of the solvent and partial imidization of the precursor is carried out until the film becomes self-supporting. The film is then stripped from the mold, the mold is replaced with the outer periphery of a tubular mold, and then the removal of the solvent and the imidization reaction is completed. When a polyamic acid solution principally containing 3,3′,4,4′-biphenyltetracarboxylic dianhydride is used, an attempt to complete the imidization after rotational molding with the film of the solution adhering to the cylindrical mold causes the film to be stripped from the inner surface of the mold due to the evaporation of the solvent, the volume shrinkage force created during the imidization reaction, and the shrinkage stress caused by a strong surface orientation during the imidization reaction. For this reason, the imidization step involves stripping the film from the cylindrical mold, and replacing the cylindrical mold with the outer periphery of a tubular mold.
However, the imidization using a tubular mold as mentioned above results in an inability to remove the residue on evaporation of the solvent remaining on the stripped belt. This has posed problems such as the expansion of the belt due to the solvent gathered between the belt and tubular mold, and a deterioration in the flatness of the belt due to the shrinkage that occurs during the imidization reaction, causing the belt to ripple.    Patent Document 1: Japanese Unexamined Patent Publication No. 5-077252    Patent Document 2: Japanese Unexamined Patent Publication No. 7-156287    Patent Document 3: Japanese Unexamined Patent Publication No. 10-63115    Patent Document 4: Japanese Unexamined Patent Publication No. 10-83122    Patent Document 5: Japanese Unexamined Patent Publication No. 2000-281902    Patent Document 6: Japanese Unexamined Patent Publication No. 2003-266454    Patent Document 7: Japanese Unexamined Patent Publication No. 2006-206778